This invention relates to gunsight systems for controlling an airborne gun platform for air-to-air gunnery which is particularly applicable, but not necessarily restricted to, controlling gunfire in air superiority fighter aircraft.
The state of the art in aircraft gunsight systems has remained relatively unchanged since the 1940's and 1950's and has typically been based on implementation of the so-called "LCOS" (Lead Computing Optical Sight) system. An example of an LCOS system is described in U.S. Pat. No. 2,467,831 issued to F. V. Johnson in 1949.
This type of system provides the pilot of the aircraft with a reticle (sometimes also called a "pipper") image on an optical head-up display (HUD) panel. Through use of collimating optics in the sight system, the image of the reticle is made to appear at infinity in the pilot's field of view. The position of the reticle on the display panel is controlled by a two-axis gyro in a manner which is dependent on the angular velocity of the line of sight to the target and the projectile time of flight to the target. Operation of the LCOS system generally requires the pilot to maneuver the attacking aircraft so that the reticle is near the target for some minimum time. At the same time, an accurate estimate of the range of the target aircraft has to be entered into the system.
When the target is being "tracked" by the LCOS reticle and an accurate target range input is available, the attacking aircraft is properly oriented so that the muzzle velocity vector of its gun (appropriately compensated for gravity drop) is in the turning plane of the target and is offset at the correct lead angle. Firing of the gun at this time maximizes the likelihood of achieving a hit on a target.
The relationship between the various system parameters which is necessary to obtain a hit is the following:
______________________________________ ##STR1## (1) where --.lambda.= vector lead angle (for sin .lambda. .congruent. .lambda.) 2 T.sub.n = sensitivity time .congruent. time of flight T.sub.f = time of flight --.multidot..beta.= angular rate of line of sight to target -a.sub.r = target acceleration relative to own aircraft acceleration -a.sub.n = own aircraft lift acceleration V.sub.f = average velocity of a bullet relative to own aircraft --S= unit vector along the line of sight f.sub.b = ballistic curvature function as exemplified in the second term on the right side of equation (13) V.sub.b = bullet initial velocity .alpha. = aircraft angle of attack T.sub.f = bullet time of flight .alpha..sub.g = gun angle of attack. ______________________________________
The lead computing optical sight (LCOS) does not have a target angle tracker and therefore implements equation (1) by developing a line of sight rate as the dependent variable, i.e.,: ##EQU1##
The lead angle, .lambda., in this case is the angle between the gun and the reticle and is equal to the target line of sight angle only if the pilot tracks the target. Excessive time is frequently required for the pilot to "settle" the reticle near enough to the target to be a useful reference in highly transient gun attacks.
Fire control systems which have attempted to mechanize equation (1) by measuring the angular rate of the line of sight to the target, .beta., with an independent tracking device (e.g. radar) and then compute the required lead angle have been called "director systems."
A critical limitation of this type of system has been the lack of accuracy of measurement of the .beta. parameter by means of the tracking subsystem. Available trackers are extremely vulnerable to target-generated noise, e.g., motion of the tracker across the target due to changes in contrast, and other noise sources such as background clutter, wave front interference effects, and the like. These phenomena are typically in the same spectral region as the actual target maneuvers being tracked, and therefore are difficult to separate by filtering techniques. For example, in the case of an electro-optical or radar tracker following a specific reflection from the target, a sudden change in target attitude will cause a sudden change in the position of the reflection, e.g., from one wing tip to the other, and the tracker will respond by generating a false indication of a rapid change in the angular velocity of the target.
Because the director system displays the reticle in terms of vector lead angle (.lambda.) pursuant to the above equation, the rate of change .beta. of the target line of sight magnifies the effect of such false tracking outputs such that the displayed reticle is very unstable. In other words, because the system responds to the rate of change of tracking noise, it is very difficult for the pilot to achieve solid registration of the pipper on the target for the amount of time necessary to ensure accurate firing. The pilot of the target aircraft is able to capitalize on this flaw in the system by performing evasive maneuvers which enhance the amount of the noise introduced into the tracking system.
The system of the present invention eliminates the first order effects of target noise by presenting the pilot with an angle rate display based on measured target angle, rather than an angle display based on measured target angular rate.